This site recently fell victim to a hack, whereby someone managed to inject some malicious code which went on to mess with large parts of my wordpress installation. The code served to redirect visitors to sites that looked innocuous enough, but contained malware.
Thankfully, I caught it very fast and the malware was pretty out of date, so my computer identified it immediately, quarantined it and then trashed it.
Unfortunately, it took me a long time to get rid of the infection. I spent all of last night working on the site, trying to figure out where the remaining parts of the malware were.
Eventually, the only solution that provided me with some confidence that the site was clean was to roll back everything to the way the site was over a week ago, including the database.